1. Field of the Invention
Natural phosphate rock, or fluorapatite, is the primary source of phosphorus for practically all phosphatic chemicals. Two processes are in common use around the world to produce phosphorus, (1) the electric furnace process, and (2) the acid, or wet, process.
In the electric furnace, the phosphate rock is melted and the phosphorus vapors are condensed as elemental phosphorus. By-products consist of calcium silicate slag, carbon monoxide, carbon dioxide and ferrosilicon, all of which have only some commercial value at present.
The acid process uses a strong mineral acid, usually sulfuric acid, to digest the rock, releasing phosphoric acid and leaving a residue of calcium sulfate, or gypsum, and small quantities of phosphorus, fluorine and various trace elements. This by-product, at present, has no commercial value and, in fact, poses an environmental problem because of the contamination of rainwater runoff from soluble compounds in the gypsum.
This invention is a process whereby all of the solid and liquid residue from the phosphoric acid manufacture is recovered as a commercially valuable product, with the option of using high sulfur low cost fuels, which cannot be used economically elsewhere, and leaving only normal and sulfur-free combustion gases as a residue. This process incorporates mostly existing equipment in a novel and unique sequence which will significantly improve the technology of phosphoric acid manufacture and process economics, eliminate an environmentally repugnant and unacceptable residue, and produce useful and much needed coproducts.
The sulfur values in the calcium sulfate and in the fuel are recovered and regenerated into the sulfuric acid used to digest the phosphate rock. The calcium sulfate after de-sulfurization is an impure lime. Some fluorine and phosphorous are recovered at a low temperature acid and water treatment of the gypsum, and most of the remaining fluorine and phosphorus, with other trace elements, are removed and recovered at high temperature in an electric furnace. The molten impure lime in the furnace is tapped, cooled, and sold as a calcium silicate material or for use in manufacturing refractory or rock wool insulation materials.
2. Description of Prior Art
The invention described below is an improvement of the conventional acid, or wet, process for phosphoric acid manufacture. As such, the improved process herein incorporates existing equipment with new technology in a novel and unique sequence and manner. The conventional wet, or acid, process for phosphoric acid produces a residue of impure calcium sulfate di-hydrate, or waste phospho-gypsum. It requires the burning of new, elemental sulfur to ultimately produce sulfuric acid in a conventional sulfuric acid contact or chamber process plant or the obtaining of fresh sulfuric acid from an off site source. The acid digests the calcium phosphate rock; the sulfur trioxide combines with calcium oxide to form a calcium sulfate, which is then discarded as having no commercial value. In fact, drainage from rainwater entering the waste stockpiles produces an acid effluent which is environmentally unsafe, and must be treated before release. Therefore this economical and technically feasible process for elimination of the waste products serves many useful purposes.
The process for manufacture of phosphoric acid from phosphate rock and sulfuric acid (wet process) is over 80 years old in essentially its present form. A complete description is generally available in textbooks and trade documents. Essentially, the process follows these steps:
(1) Calcium phosphate rock, or fluorapatite, is mined, beneficiated by washing and sintering, and shipped to the phosphoric acid plant.
(2) The phosphate rock is ground to a fine powder in a dilute solution of phosphoric acid. The slurry is passed into digester tanks where it is reacted with 55% sulfuric acid, diluted with phosphoric acid. Water vapor, carbon dioxide, and fluorine compounds are evolved as gases; the fluorine values are recovered by absorption.
(3) Acid digestion of the slurry requires 4 to 8 hours at 75.degree. to 80.degree. C. for the usual di-hydrate process, or at a slightly higher temperature for the hemi-hydrate (HDH) process. The objective is to form easily-filtered and easily washed crystals of the di-hydrate or hemi-hydrate form of calcium sulfate. The calcium in the phosphate rock reacts with sulfate from the sulfuric acid to form the calcium sulfate. The hydrogen ions from the sulfuric acid combine with phosphorus to form phosphoric acid and with fluorine to form hydrogen fluoride.
(4) The acid-gypsum slurry from the digester tanks is taken to acid recovery, usually some form of a vacuum filter. The phosphoric acid is removed leaving a gypsum cake, in which the solids are calcium sulfate di-hydrate or hemi-hydrate with about 5 to 10% impurities, mostly iron oxide, alumina, and silica, derived from the parent rocks.
(5) The phosphoric acid filtrate is taken to an evaporator, or concentrator, where it is evaporated to the desired concentration. Fluorine and phosphorus values are recovered from the evaporated fumes.
(6) The impure gypsum crystals are washed with water and filtered. The first filter waters are returned to the digester tanks and later, more diluted water is taken to a wastewater treatment plant where the acidity is neutralized. The washed cake is slurried with wastewater and sent to a settling basin; the water is returned to the process and the settled phospo-gypsum is taken to a stockpile as a waste product.
(7) If desired, the phosphoric acid from step (5) above may be treated by one or another of several patented processes to recover uranium values using the "yellow cake" process.
(8) The sulfuric acid used in step (2) above, to digest the phosphate rock, is usually obtained from an intra-plant sulfuric acid plant. This may be a chamber process plant or the more common contact process plant, using a vanadium oxide catalyst. Such plants are of conventional and/or proprietary design and are not claimed in this invention. Most plants use sulfur dioxide from a convenient and economical source to form into sulfur trioxide to form into sulfuric acid. If a contact process plant is used, the catalyst is poisoned, or inhibited from functioning, by trace amounts of impurities such as chlorine, fluorine, and other elements. Therefore, in such plants it has been common practice to obtain sulfur dioxide gas by burning elemental, pure sulfur.
Prior to this invention there have been many attempts to devise processes to recover all or part of the economically recoverable products contained in the waste phospho-gypsum and to concurrently solve the waste disposal problem. In Europe and other parts of the world, the phospho-gypsum is disposed of by dumping in the open sea, with loss of all commercial value. In other areas, notably the United States, disposal must be in a storage pile or impounding basin. The leaching action of rainwater and/or storage water produces an acidic effluent that may enter the nearby surface and/or groundwater regime, creating an environmental hazard. Increasingly stringent regulations require the collection of the effluent waters, and neutralization. This is an expensive and non-productive process.
The technical literature of the last 40 years or more includes discussions of and patents for various methods for the commercial use of all or part of the constituents of phospho-gypsum. In Japan, the United Kingdom, and several other countries, where natural gypsum is in short supply, the phospho-gypsum has been economically converted to plaster products, such as Plaster of Paris, to gypsum wallboard, or as an additive to portland cement, acting as a set retarder. This is not economically feasible in places such as North America where natural gypsum abounds.
Various proposals have been made for conversion of phospho-gypsum to useful and economical products by chemical conversion. In every instance, although technically feasible, the cost of the chemicals to cause the conversion has been greater than the value of the resulting product. An example is the reaction of gypsum with ammonia and carbon dioxide to form ammonium sulfate and calcium carbonate. Because of its low purity compared to natural gypsum, and that the production of urea from the ammonia and carbon dioxide forms a higher value product, the use of phospho-gypsum has not proven economical in this manner.
It has also been recognized, in the technical literature, that the recovery of sulfur from phospho-gypsum, to form sulfuric acid, can be possible in certain instances, particularly in light of the increasing cost of recovery of natural sulfur by mining. This method involves calcination of the impure gypsum leaving a calcium silicate co-product. It has generally been concluded that this process is not completely technically and economically feasible because of the high cost of drying and calcining the gypsum, the corrosive effect of the hydrofluoric acid formed in the off gases and the detrimental presence of phosphorus in the calcium silicate. It is significant that almost all processes of this type utilize a rotary kiln as the drying-calcining-fusing device. That device is notoriously heat inefficient and does not provide a means for technically and economically recovering the sulfur, phosphorus, and/or the fluorine values. The present invention uses an electric furnace for the final, high temperature stage of fusion which liberates the remaining phosphorus and other values in a separate and inert or controlled atmosphere, permitting collection of the phosphorus and other vapors.
At no place in the existing technical literature is there a description or suggestion of a total process involving acid washing the phospho-gypsum waste product, calcining the washed product to re-cycle the sulfur values, and fusing the remaining product in an electric furnace to recover residual phosphorus and fluorine values, leaving a calcium silicate of commercial value for use in other manufacturing processes such as the refactory or rock wool insulation industries. This is the claim of the present invention.